The Collaborative International Dictionary of English v.0.48:

Serum \Se"rum\ (s[=e]"r[u^]m), n. [L., akin to Gr. ???, Skr.
   s[=a]ra curd.] (Physiol.)
   (a) The watery portion of certain animal fluids, as blood,
       milk, etc.
   (b) A thin watery fluid, containing more or less albumin,
       secreted by the serous membranes of the body, such as the
       pericardium and peritoneum.
       [1913 Webster]

   Blood serum, the pale yellowish fluid which exudes from the
      clot formed in the coagulation of the blood; the liquid
      portion of the blood, after removal of the blood
      corpuscles and the fibrin.

   Muscle serum, the thin watery fluid which separates from
      the muscles after coagulation of the muscle plasma; the
      watery portion of the plasma. See Muscle plasma, under
      Plasma.

   Serum albumin (Physiol. Chem.), an albuminous body, closely
      related to egg albumin, present in nearly all serous
      fluids; esp., the albumin of blood serum.

   Serum globulin (Physiol. Chem.), paraglobulin.

   Serum of milk (Physiol. Chem.), the whey, or fluid portion
      of milk, remaining after removal of the casein and fat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

serum
    n 1: an amber, watery fluid, rich in proteins, that separates
         out when blood coagulates [syn: serum, blood serum]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "serum":
   Rh factor, Rh-negative, Rh-positive, Rh-type, Rhesus factor,
   agglutinin, allergen, anaphylactin, antiantibody, antibody,
   antigen, antiserum, antitoxic serum, antitoxin, antivenin,
   arterial blood, arterial transfusion, blood, blood bank,
   blood cell, blood count, blood donor, blood donor center,
   blood group, blood grouping, blood picture, blood platelet,
   blood pressure, blood serum, blood substitute, blood transfusion,
   bloodmobile, bloodstream, chyle, circulation, clinical dextran,
   colostrum, dextran, direct transfusion, discharge,
   drip transfusion, erythrocyte, exchange transfusion,
   exsanguination transfusion, exsanguino transfusion, gleet,
   globulin, gore, grume, hematics, hematologist, hematology,
   hematoscope, hematoscopy, hemocyte, hemoglobin, hemometer, humor,
   ichor, incomplete antibody, interferon, isoantibody, lachryma,
   lactation, leukocyte, leukorrhea, lifeblood, lymph, matter, milk,
   mucor, mucus, neutrophil, opsonin, peccant humor, perfusion,
   phagocyte, phlegm, plasma, plasma substitute, plasma transfusion,
   precipitin, purulence, pus, reciprocal transfusion, red corpuscle,
   replacement transfusion, rheum, saliva, sanies, serous fluid,
   serum transfusion, snot, suppuration, sweat, tear, teardrop,
   the whites, transfusion, type O, urine, venous blood,
   venous transfusion, white corpuscle